Reverse currents are defined as currents flowing in the opposite direction to the currents generated by the photovoltaic generator in normal operation.
Where a number of strings of a photovoltaic generator are connected in parallel to a common DC link, a reverse current may arise e.g. if an individual string is in shadow, and thus the intermediate circuit voltage delivered by the other, unshadowed strings of the DC link causes a reverse current to flow through the shadowed string.
The constituent photovoltaic modules in the strings of a photovoltaic generator will not be damaged by small reverse currents. However, where a plurality of strings are connected in parallel, the risk will arise that an individual string which is in shadow or which, for other reasons, delivers a significantly lower output voltage than the strings connected in parallel, will receive currents from all the remaining strings in the form of reverse currents. The string concerned will rapidly be overloaded as a result.
If a string with an incorrect polarity is connected on a common bus connection with a number of other strings, it will short-circuit the other strings connected on the bus connection. The resulting short-circuit current, i.e. the sum of the currents generated by the other strings, will not flow back through the string connected with an incorrect polarity, but will therefore flow counter to the forward direction of a correctly connected string. In any case, the high short-circuit current in all the other strings will constitute a potential source of damage to the string connected with an incorrect polarity. The overloading of the affected string may also cause further damage—specifically, the outbreak of fire may occur as a result of the overheating of components in the affected string.
From EP 2 284 973 A1, a photovoltaic installation with a plurality of strings is known, each of which is comprised of a number of photovoltaic modules connected exclusively in series. The strings are connected to one another in parallel on bus lines. A transformer feeds electrical energy from the bus lines into a grid system. By the regulation of the transformer, it is possible to adjust the voltage released between the bus lines. Each string is provided with a current sensor which, as a minimum requirement, detects whether a reverse current is flowing to the string, and indicates said reverse current to the control system. The control system reduces the voltage between the bus lines, thereby interrupting the reverse current. Accordingly, the individual strings are not provided with a reverse current-blocking diode. The current sensors are preferably arranged in a number of decentralized terminal units. A number of strings in the field are connected in parallel to the terminal units by means of a pair of connecting cables, and these connecting cables are then routed to a central unit which incorporates the transformer. The central unit is provided with terminals for the connection of the central terminal units to the bus lines.
From EP 2 282 388 A1, a device is known for the in-feed of electrical energy from a plurality of strings of photovoltaic modules into a grid system. The device is provided with a connection for each string, which incorporates means for overcurrent protection and for the selective tripping of the string. Each string is customarily comprised, not only of a series circuit of photovoltaic modules, but of a regular arrangement of multiple sub-strings, which are combined to form a parallel circuit. Means for overcurrent protection and for the selective tripping of individual strings are each provided with a power circuit-breaker, with a motor-driven opening and closing system, connected in series to a current sensor between the relevant string and a bus line. The bus line is connected to an inverter.
From DE 101 20 595 A1, a solar energy system comprising a standard solar cell chain and a sub-standard solar cell chain is known. The DC voltage delivered by the sub-standard solar cell chain is raised to the level of the DC output voltage from the standard solar cell chain by means of a booster unit. The DC voltage from the standard solar cell chain and the raised DC voltage are fed to an inverter, which generates an AC voltage which is then delivered to an electric power supply system. A reverse current-blocking diode is arranged respectively between the standard solar cell chain and the booster unit on one side, and the inverter on the other side. The booster unit may be configured as a voltage-raising circuit, i.e. as a boost converter.
From DE 10 2009 032 288 A1, a photovoltaic installation is known. The photovoltaic installation is comprised of photovoltaic modules, which are connected to form a number of module strings, and are protected against reverse currents. The module strings are connected to dedicated DC/DC converters, and the outputs from the DC/DC converters are fed to the input of a common inverter. The DC/DC converters are arranged in at least one generator terminal box, which is physically separated from the inverter. One or two module strings may be connected to each DC/DC converter. Where two module strings are connected per DC/DC converter, string protection facilities may be omitted. Where only one module string is connected per DC/DC converter, string diodes may also be omitted, as the galvanic isolation function delivered by the DC/DC converters inhibits any reverse current. As no further control function is available on the galvanically-isolating DC/DC converters, the current can be prevented by the DC/DC converters. Any additional DC isolating point may be omitted accordingly. Moreover, the individual module strings may be disconnected, without disconnecting the entire photovoltaic installation.
From US 2007/0107767 A1, a DC generating system is known, comprised of a number of strings of energy-generating cells connected in parallel to bus lines. The current flowing through each string is measured by a current sensor. Each string is also provided with a switching module, for the connection of the relevant string to the bus lines, the disconnection or short-circuiting thereof by means of a dynamic load.